Hydraulic fracturing is a well known process of pumping a fracturing or “fracing” fluid into a wellbore at an injection rate that is too high for the formation to accept without breaking. During injection the resistance to flow in the formation increases, the pressure in the wellbore increases to a value called the break-down pressure, that is the sum of the in-situ compressive stress and the strength of the formation. Once the formation “breaks down,” a fracture is formed, and the injected fluid flows through it. From a limited group of active perforations, ideally a single, vertical fracture is created that propagates in two “wings” being 180° apart and identical in shape and size. In naturally fractured or cleated formations, it is possible that multiple fractures are created and/or the two wings evolve in a tree-like pattern with increasing number of branches away from the injection point.
Fluid not containing any solid (called the “pad”) is injected first, until the fracture is wide enough to accept a propping agent. The purpose of the propping agent is to keep apart the fracture surfaces once the pumping operation ceases, the pressure in the fracture decreases below the compressive in-situ stress trying to close the fracture. In deep reservoirs, man-made ceramic beads are used to hold open or “prop” the fracture. In shallower reservoirs, sand is normally used as the propping agent.
Typically fracturing fluids used for well stimulations consist primarily of water but also include a variety of well known additives, such as a hydrated gel that is used for carrying the proppant and other components that allow the fracturing fluid and the proppant to be pumped to a target zone at a high rate and reduced pressure. The gel is hydrated prior to being introduced into the fracturing fluid system. The gel's hydration is sufficient to give it adequate viscosity to support the proppant during the injection process. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture. The gelation or viscosifying of polymers and/or surfactants in the fracturing fluid typically obtains the requisite viscosity. The selection of additives is based on a number of factors including the makeup of the formation, formation fluids, pumping configurations, and borehole temperatures without breaking down.